The present invention relates to digital communication systems and more particularly to digital communication systems employing multi-stage transceivers.
A point to multipoint wireless communication system represents a potentially effective solution to the problem of providing broadband network connectivity to a large number of geographically distributed points. Unlike optical fiber, DSL, and cable modems, there is no need to either construct a new wired infrastructure or substantially modify a wired infrastructure that has been constructed for a different purpose. The point to multipoint system includes both a hub or a head end and numerous subscriber units associated with individual residences or businesses. For both the subscriber unit and head end, there is a transceiver including both a transmitter and a receiver. Both the transmitter and receiver incorporate various stages. Special problems arise in signaling among the stages as will be explained.
One problem arises in passing signals between physically distinct components of the subscriber unit. For optimal network performance, it is desirable that the subscriber unit employ an outdoor antenna. The source and sink of data at a business or residence will however likely be a computer located inside a building. If the subscriber unit were to be integrated within one physical package inside the building in proximity to the computer, a problem arises in that RF signals will attenuated by the cable between the indoor subscriber unit housing and the outdoor antenna. If the subscriber unit is a single physical package outside the house at the antenna location, another problem arises in that high speed unmodulated data passing back and forth between the computer and subscriber unit may be corrupted over the relatively long distance between the subscriber unit and computer.
Therefore, it is desirable to divide the subscriber unit components between an indoor unit and an outdoor unit. The outdoor unit includes components that operate at microwave frequencies. The indoor unit performs packet processing, baseband signal processing, and processing at an intermediate frequency (IF) suitable for low-loss coupling between the indoor unit and outdoor unit. The outdoor unit can then convert between the IF signal and the RF signal at microwave frequency.
Numerous signals, however, must pass between the indoor unit and outdoor unit including the transmitted and received IF signals, a frequency reference signal, frequency adjustment information, and power control. Yet it is impractical to pass multiple cables between the indoor unit and outdoor unit because this would greatly complicate installation and maintenance.
Another interstage signaling problem arises at the head end. The head end needs to regulate the upstream transmission power level used by each subscriber unit. If this transmission power level is too low, the subscriber unit signal received at the head end will be too weak to accurately recover the transmitted digital data. If this transmission power level is too high, the subscriber unit signal may saturate the head end receiver or cause interference to communication links operating outside network 100. For example, network 100 may be a single cell of a multicellular system and excessive transmission power may cause interference to other cells.
To regulate subscriber unit transmission power, the head end will typically pass power control commands to subscriber units. The power control commands are determined based on a received signal level measurement made within the head end receiver. This measurement is made within the head end receiver after several stages of receiver processing including RF filtering, downconversion to an intermediate frequency, and various other filtering and processing stages. However, the signal gain through these premeasurement components will vary over time and among head end units. Because recovery of the transmitted data occurs after the measurement point, the variation of this signal gain and the resulting variation of subscriber unit transmission power will not affect head end receiver power, however, an increase in subscriber unit transmission power due to receiver gain changes rather than channel conditions will potentially cause interference to cells outside network 100.
It is therefore desirable to measure the head end receiver gain and compensate for it in the operation of the subscriber unit power level control loop. However during operation it is difficult to accurately measure head end receiver gain because although the signal level at the measurement point is known, there is no easy way to measure the signal level incident at the antenna.
What is needed are systems and methods for interstage signaling that address the problems noted above.